Improved firearm

ABSTRACT

An improved firearm includes in some embodiments a gas block of an indirect impingement system mounted on the barrel near the firing chamber to provide gas at a higher pressure for moving the piston that moves the bolt carrier. Some embodiments include low frictions rails on which the bolt carrier moves within the upper receiver. The higher gas pressure and the reduced friction from the rails reduce the required stroke of the piston, allowing a shorter firearm to be constructed. In some embodiments, mating elements, such as pins and corresponding openings, on the upper receiver and the lower receiver, align and provide additional structural strength to the upper and lower receiver.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to improvements on the design and operation of firearms.

BACKGROUND OF THE INVENTION

Autoloading rifles are popular with military, police, and civilian shooters. Of the various autoloading rifle designs, few can compare with the popularity of the M16 family of firearms and its derivatives. The M16 family of firearms includes, but is not limited to, the AR15, M4, AR10, SR25, and others. Firearms typically include a receiver that houses several working components of the firearm, including firing components, with a barrel extending from the receiver. The M16 family of firearms typically includes a lower receiver that has a stock coupled to the rear end and that is connected to an upper receiver having a barrel coupled to its front end. The lower receiver typically houses the trigger assembly and provides a magazine well, into which a magazine containing ammunition may be inserted. Such firearms will be referred to herein as M16-type firearms.

The upper receiver is secured to the lower receiver by two metal pins, each pin inserted through holes in lugs in the bottom of the upper receiver, the pins extending through holes in the lower receiver. M16-type firearms are typically manufactured with relatively loose tolerances to allow fitment between products from different manufacturers, which can allow significant misalignment and lateral play between the upper and lower receiver. The play may be further exaggerated by wear to the lugs, the corresponding aligned openings, and the locking pins. This play is unacceptable when precision shooting is required.

The upper receiver takes the form of a tubular structure having a substantially circular cross section and an open bottom section where the upper receiver mates with the lower receiver. Due to this open tubular structure, the upper receiver is relatively weak in the lateral direction. When subjected to a crushing force, the tubular structure tends to deform, causing the cross section of the upper receiver to change from approximately circular to oval, rendering the firearm inoperable.

The upper receiver defines an internal passage in which the bolt carrier reciprocates between a firing position, from which a live round of ammunition can be fired, and a retracted position, from which a spent casing is ejected. The bolt carrier typically includes raised bosses on the surface that contact internal bearing surfaces of the receiver housing. Play between the contacting components allows misalignment of the bolt assembly with the receiver housing, increasing wear and misalignment of the bolt carrier as it cycles.

The bolt carrier assembly and receiver housing are subjected to high internal temperatures due to the hot gases used for weapon cycling and to friction between the bolt carrier and the bearing surfaces on the receiver. Heat from the hot gases and friction can burn off or degrade essential lubricants. The firearm components are also subject to non-linear cycling that causes stress and vibration. These stresses can be compounded by environmental factors such as sand, dirt or other foreign material, moisture, fouling from gunpowder residue or degraded lubricant, and neglect, all of which can result in failure of the action to cycle properly or increased inaccuracy of the firearm. These problems are especially evident during automatic or rapid semi-automatic firing. Current strategies to combat these factors involve rigorous cleaning, lubrication, and other maintenance, which are time consuming and must be performed often.

Even a cleaned and well lubricated firearm can suffer from failures due to degradation of lubricants within the action of the firearm. This problem can occur with firearms which are used infrequently, such as those in the arsenal of a police department. If an entity possesses many such firearms, the maintenance required to ensure reliable operation can be a substantial burden in terms of time and expense.

There are various methods for providing the force to move the bolt carrier, causing it to eject spent casings and to load a fresh cartridge from the magazine. Typically, the M16 family of firearms uses a direct impingement system for cycling of the bolt carrier. In a direct impingement configuration, a small portion of combustion gas is bled from the barrel into a gas block and travels rearward through a gas tube. The gas exiting the gas tube impinges directly on the bolt carrier, driving it to the retracted position, from which the spent casing is ejected. The bolt carrier then returns to the firing position under a force applied by the bolt carrier return spring.

In contrast to direct impingement systems, indirect impingement typically employs a piston connected to a pushrod to cycle the bolt carrier when the firearm is fired. Two types of indirect impingement systems exist: long-stroke and short-stroke. In long-stroke systems, the pushrod is mechanically fixed to the bolt group, so that the piston and pushrod travel the same distance as the bolt group. In a short-stroke system, as is typically used in M16-type firearms equipped with an indirect impingement system, the push rod is not permanently attached to the bolt group. The energy to cycle the bolt carrier is imparted in a high-energy push over a short distance, after which the piston group stops its rearward travel while the bolt group continues to travel towards the rear of the firearm and completes the cycle using kinetic energy imparted during the brief push. The bolt carrier then returns to the firing position as in a direct impingement system.

Direct impingement systems were historically preferred for M16-type firearms due to weight savings and fewer moving parts. Typical indirect impingement systems for M16-type firearms mount the piston and pushrod in the same location as the gas tube in direct impingement models. Such a configuration allows the use of existing parts made for a direct impingement firearm. M16-type firearms are available from various manufacturers with both piston-operated indirect impingement and direct impingement operation. In addition, conversion kits exist to convert between the types.

A chief disadvantage to direct impingement systems is the introduction of hot combustion gases into the upper receiver and the bolt carrier. These hot gases deposit fouling, which results from combustion of the propellant, throughout the upper receiver and bolt carrier. This fouling reduces reliability of the firearm, requires frequent cleaning and inspection, and shortens the time required between servicing. In addition, the hot combustion gases heat the bolt carrier and upper receiver, causing essential lubricants to burn off or degrade, reducing lubrication and further exacerbating the issues mentioned before. The fouling deposited further increases friction between the dissimilar metals of the bolt carrier and the upper receiver, generating additional heat as the firearm cycles the bolt carrier. The lubrication and potential fouling deposited on the bolt carrier also tends to accumulate additional environmental debris, such as sand, dirt, or dust to accumulate, compounding the problems.

An indirect gas impingement system does not route exhaust gases back to the bolt carrier, which reduces fouling. Indirect impingement designs, however, increase the number of components, causing an unwanted increase in the weight of the firearm and serviceability problems with the more complex mechanism. The heavy gas block and piston system are typically mounted near the muzzle, resulting in a firearm with more weight distributed forward along the barrel, increasing the moment of inertia of the firearm. This can accelerate operator fatigue when holding the firearm in a horizontal position, such as when aiming and firing, as well as reducing the operator's ability to quickly pivot the firearm to engage potential targets.

The piston stroke required restricts the location of the gas port in existing M16-type firearms, which limits the amount which the barrel can be shortened while still allowing enough gas to exit the gas port to cycle the bolt carrier.

The indirect impingement system has a relatively low tolerance for variation in the pressure of the exhaust gas used for cycling the firearm. Gas pressures that are too low may fail to provide sufficient energy during operation, leading to incomplete cycling of the bolt carrier and subsequent failure to eject the spent casing or to load a fresh cartridge. Gas pressures which are too high can cause mechanical failure of various components in the cycling parts or the firearm in general, possibly resulting in injury to the user or those nearby. In the prior art, the gas block, which receives exhaust gas from the barrel, is mounted toward the muzzle to provide an adequate piston stroke. In this position, the gas pressure varies widely between various types of ammunition. The sensitivity to gas pressure variation reduces the ability of the weapon to use cartridges with varying powder loads and bullet weights, which produce varying exhaust gas pressures at the gas block.

A typical prior art solution to this problem involves the use of an adjustable gas valve mounted before the piston in the gas path. The adjustable valve allows the flow of gas to be partially or completely restricted, altering the pressure of the gas impinging on the piston. However, this system requires trial and error to determine the correct setting for a particular type of ammunition, as well as increasing complexity in the system and providing an additional point for failure of the firearm.

Recognizing these limitations in firearms of the M16 family, there is a need for improvements to the design and operating principles of firearms in this family, specifically addressing reliability, especially in adverse conditions, versatility in ammunition used without adjustments being made, and operator safety when operating such a firearm.

SUMMARY OF THE INVENTION

An object of the invention is to provide an improved firearm.

An improved firearm includes in some embodiments a gas block of an indirect impingement system mounted on the barrel near the firing chamber to provide gas at a higher pressure for moving the piston that moves the bolt carrier. Some embodiments include low frictions rails on which the bolt carrier moves within the upper receiver. The higher gas pressure and the reduced friction from the rails reduce the required stroke of the piston, allowing a shorter firearm to be constructed. In some embodiments, mating elements, such as posts and corresponding openings, on the upper receiver and the lower receiver, align and provide additional structural strength to the assembly comprising the upper and lower receiver.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows an exploded view of an upper and lower receiver;

FIG. 2 shows a front view of a lower receiver;

FIG. 3 shows a front view of an upper receiver;

FIG. 4 shows a front view of mated upper and lower receivers;

FIG. 5 shows a perspective view of partially mated upper and lower receivers;

FIG. 6 shows a front view of an upper receiver and bolt carrier;

FIG. 7 shows a skeletonized view of an upper receiver.

FIG. 8 shows a perspective view of an upper receiver.

FIG. 9 shows a front perspective view of a bolt carrier.

FIG. 10 shows a rear perspective view of a bolt carrier.

FIG. 11 shows a side view of a truncated and partially skeletonized bolt carrier and barrel.

FIG. 12 shows a perspective view of a firearm.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The exemplary firearms described below provide improvements in the mounting of the upper receiver to the lower receiver and in the cycling operation of a bolt assembly within the upper receiver. The improvements facilitate using a wide variety of ammunition without adjustment or modification. The firearms described below include several novel aspects, and not every aspect need be present in every embodiment. Moreover, many of the aspects of the described embodiments may be separately patentable. The embodiments will vary greatly depending upon the specific application, and not every embodiment will provide all of the benefits and meet all of the objectives that are achievable by the invention.

As discussed herein, the term “forward” is taken to mean towards the muzzle of the firearm, and “rear” or “rearward” is taken to mean towards the buttstock of the firearm.

FIGS. 1-5 show portions of a firearm using additional mating elements, such as posts and corresponding holes, on the upper receive and the lower receiver to provide additional mechanical strength to the assembly and better alignment of the parts. FIG. 1 shows an exploded perspective view of receiver 102 of a M16-type firearm, comprising an upper receiver 104 and lower receiver 106. The upper receiver 104 is generally tubular in structure, and features an attachment area 108 for optics or gunsights, and presents a threaded area 110 for attachment of a barrel (not shown). The upper receiver 104 connects to the lower receiver 106 via mounting lugs 120 and 126.

The lower receiver 106 provides a threaded area 128 for attachment of a buttstock and defines a magazine well 112 for insertion of a magazine (not shown) containing ammunition for the firearm. The lower receiver 106 also houses the trigger group (not shown) and the firearm's safety mechanism (not shown). The lower receiver has front openings 118 in lugs 116 and rear openings 124, through which pins 114 and 122 are inserted to secure the upper and lower receiver together when mated. Removal of rear pin 122 without removal of front pin 114 allows the upper receiver to pivot around front pin 114 without separating the two sections of the firearm.

In some embodiments, additional mating structural elements are provided to connect the upper receiver 104 and the lower receive 106. For example, removable posts 134 and 136 are inserted into openings 126 and 132 in lower receive group 106 and corresponding opening (not shown) in the upper receive 104

Connecting the upper receiver 104 and lower receiver 106 with such posts and corresponding aligned openings, or other mating elements, increases the structural strength and improves the accuracy of the alignment. In the prior art, the upper receiver takes the form of a hollow tube in which a section has been removed from the bottom to allow mating with the corresponding flat top section of the lower receiver. The prior art upper receiver is therefore relatively weak and the tube is subject to deformation. The addition of posts and corresponding aligned openings in the opposing receiver allows relatively weak sections of the upper receiver to transmit a portion of a stress load to the comparatively strong section on the corresponding lower receiver. This load sharing allows increased rigidity and strength, for example, preventing deformation of the tubular character of the upper receiver if the firearm is accidentally run over by a vehicle.

In addition, by providing positive mating of the upper and lower receiver at locations other than at the lugs 116, increased lateral rigidity is achieved. For applications where the firearm is secured via the lower receiver to a structure, such as a bench rest, even a small shift of the alignment of the barrel with respect to the lower receiver can cause unacceptable inaccuracy issues or require repositioning of the firearm between firing subsequent rounds. Lateral play is also regarded as annoying by operators and leads to doubts about the quality of the firearm construction. The aligned slots and posts may be of any number, shape, or size to accommodate various design considerations of the upper or lower receiver, and can extend from the upper receiver or the lower receiver.

The posts or other mating elements can be permanently affixed to or integral with the upper receiver 104 or the lower receiver 106, with a mating opening in the opposite one of the upper or lower receiver. FIG. 2 shows a lower receiver 202 having monolithic post 204 and a slot 206. By “monolithic post” is meant a post that is not removable. The monolithic post may be a manufactured into the receivers or permanently attached, such as by an interference fit. A mating upper receiver would include a slot that mates with monolithic post 204 and a monolithic post or a removable post that mates with slot 206. In some embodiments, the post is formed during manufacturing of the upper receiver or lower receiver.

Using removable posts as shown in FIG. 1 facilitates mating of an upper or lower receiver having or equipped to accept posts to a corresponding receiver which is not equipped with posts or aligned openings, such as a standard upper or lower receiver of the prior art. In both removable and permanent post systems, the posts, aligned openings, or both are preferably arranged on the receiver such that the posts or aligned slots do not interfere with conventional mating with an upper or lower receiver which does not have the corresponding posts or aligned slots. This may be achieved by, for example, mounting the posts to the outside of the frame, as shown in FIG. 1.

FIG. 3 shows an upper receiver 302 with slots 304 for accepting posts protruding from a lower receiver. FIG. 4 shows mated receivers 402 comprising upper receiver 404 and lower receiver 406. The lower receiver has a monolithic post 410 fitting into a slot in the upper receiver. The mated receivers also utilize a removable post 412, fitting into a slot 414 in the upper receiver. After mating the upper and lower receiver, the mated receiver may be retained in the mated configuration by insertion of a pin 408 into an aligned opening 414. The retention lugs and pins may be similar to those used in conventional firearms.

FIG. 5 shows partially mated receivers 510, comprising upper receiver 502 and lower receiver 504. Inserted into the lower receiver are forward posts 506 and rear posts 508. As shown in FIG. 5, the firearm may include a four sets of mating elements, with sets of mating elements on opposite sides across the receiver and spaced apart along the longitudinal axis of the receiver. The invention is not limited to any particular quantity, position, or type of mating elements.

In some embodiments, the bolt carrier is supported and guided during cycling by one or more rail members. Rail members may provide positive alignment, reduction of sliding or static friction, relative resistance to heat and contamination, reduced need for lubrication and maintenance, and/or increased durability and reliability. Rails also reduce the wear on the upper receiver and the bolt carrier.

As shown in FIGS. 6-8, rails 608 are positioned between the upper receiver 604 and the bolt carrier 606. FIG. 6 shows a bolt carrier 606 that slides on rails 608 inside the upper receiver 604. Rails 608 are fixed within the upper receiver, while the bolt carrier has grooves to receiver the one or more rails. The rails 608 position the bolt assembly radially within the upper receiver, keeping the bolt carrier accurately aligned in the receiver. While three rails are shown, a system could use one rail, two rails, four rails, or any number of rails. While the rails shown extend approximately the length of the bolt carrier, the rails could be shorter, longer and could be discontinuous, includes gaps. While circular cross sections are shown, the rails could have any cross section. For example, the grooves in the could be eliminated by using rails having concave surfaces that matches the outer cylindrical surface of the bolt carrier.

FIG. 7 depicts an upper receive 702 with parts of the receiver shown as transparent to reveal interior details. Removable posts 710 provide structural strength to the receiver when mated to a lower receiver as previously described. Barrel attachment threads 708 at the front of the upper receiver allow the attachment of a barrel. Rails 712 extend longitudinally in the tubular cavity. In some embodiments, the rails fit into slots or grooves machined into the upper receiver, allowing only a portion of the rail diameter to extend into the longitudinal tubular cavity within the upper receiver, as shown in FIG. 6.

FIG. 8 shows a perspective view of an upper receiver 702 having such grooves 804. The groves 804 have an end wall 806, which limits forward travel of the rails 712. Rails 712 are positioned such that they do not impede the normal operation of the firearm, for example, by blocking the ejection port 802.

FIG. 9 shows a bolt carrier 606 equipped with rails 902. The bolt carrier 606 serves to carry the bolt 910. The bolt is rotated from a locked position when the bolt carrier is in the firing position to an unlocked position by bolt cam 906 as the bolt carrier moves rearward during cycling.

In M16-type bolt carriers, energy for cycling the bolt carrier from the firing position to the retracted position is supplied through a carrier key 904. In some embodiments, the carrier key may take the form of a gas key, for receiving high pressure gas from a direct-impingement system. In other embodiments, the carrier key receives an impulse from a piston, such as found in an indirect impingement system. Also visible is the charging handle 908, which can be used to manually cycle the bolt carrier. FIG. 10 provides a different perspective view of the bolt carrier of FIG. 9.

The rails 712 or 902 are preferably composed of a low-friction, high-temperature material. For example, HyComp LLC of Cleveland, Ohio, sells self-lubrication bearing materials under the product name H310® which is a blend of high temperature thermoset polymer and chopped carbon fiber. Other HyComp LLC® products, such as H310®, WearComp®, WearComp 200®, FiberComp® could also be used, as could other products including graphene or a graphene composite. Rails 712 or 902 may also be composed of metal or alloys, molybdenum, titanium, or alloys thereof, or synthetic, or semi-synthetic organic solids. Low-friction rails are preferred because they can substantially reduce or eliminate the need for lubrication of the bolt carrier.

Rails can be permanently affixed to the upper receiver, to the exterior of the bolt carrier, or the rails can be “floating” in the grooves without being fixed to either the upper receiver or the bolt carrier. The rails may be readily removable to be replaced semi-consumable components of the upper receiver, and can be removed for replacement. Rails can be attached using fixtures, such as any combination of: posts, bolts, screws, pins, clips, etc.

The energy required to cycle the bolt carrier from the firing position to the retracted position is reduced if friction is reduced between the bolt carrier and the interior surface of the upper receiver. Reducing the energy required to cycle the bolt carrier provides multiple benefits. It reduces “double recoil,” in which the user experiences an initial recoil from the firing of the firearm and then a second recoil resulting from the bolt carrier slamming backwards to the retracted position. Reducing the energy also allows for the use of lighter components because the materials are subjected to less stress. Also, reducing the force required to cycle the bolt carrier reduces the pressure and volume of the gas required to push back the bolt carrier. In some embodiments, a short-stroke near-breech gas piston system is used to provide the energy to cycle the bolt carrier from the firing position to the retracted position, from where it returns to the firing position under biasing force from the buffer spring.

FIG. 11 shows such a short-stroke near-breech gas piston system in an M16-type firearm. In operation, a cartridge (not shown) is loaded in chamber 1126 by pulling back and then releasing charging handle 908 (FIG. 9), thereby manually cycling the bolt carrier. Immediately after firing, the bullet begins to travel down the bore 1124 of the barrel 1134, propelled by high-pressure gas resulting from the combustion of the propellant in the cartridge. Proximal to the forward end of the chamber, a transverse bore in the barrel defines a gas port 1122, allowing high-pressure gas to travel through a gas path bore 1130 to a gas block 1132. The high-pressure gas enters a cylinder bore 1118 and impinges on piston face 1128 of gas piston 1114, which comprises a smaller diameter portion 1114A that fits within gas block 1132 and a larger diameter portion 1114B that does not fit within gas block 1132. The forward travel of piston 1114 is limited by the increased diameter. In some embodiments, smaller diameter portion 1114A has a recessed area enlarging the volume of the gas piston so that gases can expand into piston portion 1114A and push against the interior wall. The flow of gas through the gas path bore 1130 can optionally be controlled by a needle valve 1120. In some embodiments, the piston is mechanically connected to the pushrod 1108, for example by an attachment pin 1112. In other embodiments, the piston and the pushrod may be a single component serving both functions.

The action of the high-pressure gas on the piston face produces a rearward force on the piston causing the piston and pushrod 1108 to travel rearward and compressing piston return spring 1110 against washer 1111, which has a fixed position within the upper receiver (not shown). Washer 1111 limits the rearward travel of piston 1114. Upon rearward movement, the pushrod engages a carrier key 1106 attached to bolt carrier 1102. When the force transmitted by the piston and pushrod is sufficient to overcome the spring force from the bolt carrier return spring (not shown) and friction between the various moving components and their housings, the bolt carrier and piston-pushrod assembly begin to move rearward, imparting kinetic energy to the bolt carrier. The bolt carrier and piston move rearwards under biasing force from gas pressure on the piston face within the piston bore until a gas exhaust port 1116 in the piston bore is uncovered, allowing the high-pressure gas to vent to the atmosphere and reducing the pressure within the piston bore acting against spring 1110. The piston-pushrod assembly rearward movement is stopped by the spring 1110 and washer 1111, while the bolt carrier continues rearward under kinetic energy to the retracted position, compressing the buffer spring and storing a portion of the kinetic energy. From the retracted position, the bolt carrier returns to the firing position under a force from the buffer spring and loading another cartridge from the magazine. This cycle may be repeated automatically or semi-automatically.

In some embodiments, the gas piston system is integral to the upper receiver. The gas block 1132 is preferably located on the barrel adjacent to the upper receiver. The gas port 1122 is positioned nearer to the breech than to the muzzle, more preferably less ⅓ of the distance from the breech to muzzle, even more preferably less than ¼ of the distance from the breech to the muzzle, and even more preferably less than ⅛ of the distance from the breech to the muzzle. The gas port 1122 is preferably within 6 inches of the front end of the chamber, more preferably within 4 inches of the front end of the chamber, even more preferably within 3 inches of the front end of the chamber, and even more preferably within 2 inches of the front end of the chamber.

Gas piston systems require a certain gas pressure at the gas port in order to provide enough force on the gas piston to completely cycle the bolt carrier. The pressure in the barrel increases rapidly after firing, and drops as the gas expands to force the bullet down the barrel. A gas port located near the muzzle receives relatively low-pressure gas. The low pressure necessitates a relatively long gas expansion region to delivery sufficient energy to cycle the bolt carrier. By positioning the gas carrier near the chamber, the gas pressure is higher, which allows for a shorter piston stroke. This allows the barrel to be shortened, providing a more compact firearm. Moreover, the rails 902 reduce friction, which reduces the force required to cycle the bolt carrier. The combination of the higher pressure from positioning the gas block near the chamber and reduced friction from rails 902 allows for an even shorter stroke and a more compact weapon. In some embodiments, the short-stroke near-breech gas piston system is configured such that the total length from the rear of the upper receiver to the front of the gas block is less than 20 inches, less than 15 inches or less than 11 inches. While the low friction rails facilitate the use of the short-stroke near-breech gas piston system, the short-stroke near-breech gas piston system can be implemented in system without rails. Similarly, the rails provide benefits independent from the short-stroke near-breech gas piston system, and can be used independently. Similarly, the mating posts and holes improve alignment and can thereby reduce friction. The mating posts and holes can be used with the low friction rails and short-stroke near-breech gas piston system, or independently.

Another advantage of the higher pressure and reduced friction is reduced sensitivity to variation in the ammunition used. Different types of ammunition generate different gas pressures within the barrel when fired, due to, for example, different masses of propellant in different types of ammunition. Some firearms included a manual adjustment which partially occludes the gas port to allow the use of various ammunition types. By disposing the gas block near the chamber, the pressure of the gas entering the gas block is higher than the pressure of a gas entering a gas block positioned near the muzzle. The use of relatively high pressure gas reduces the effects of pressure variation caused by different types of ammunition. This effectively allows the use of a wider variety of ammunition without manual adjustment to the gas system.

A short-stroke near-breech gas piston system is used in combination with an upper receiver having low-friction rails to further reduce the amount of energy required to cycle the bolt carrier, allowing for a very short stroke. A short-stroke near-breach gas piston system can be used in combination with an upper receiver having low-friction rails, the upper receiver also having posts or aligned slots for improving structural integrity of the firearm. The rigidity provided by the posts and openings shown in FIGS. 1-5 also reduces friction by reducing misalignment between the bolt carrier and the upper receiver.

A short-stroke near-breach gas piston system can be provided as a retrofit to an existing M16-type firearm. While the embodiment described above relates to the M16 family of firearms, the invention can be applied to other types of firearms.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

We claim as follows:
 1. An improved autoloading firearm, comprising: a receiver, comprising an upper and lower receiver, said upper and lower receiver having mating surfaces and being separable from each other, the upper receiver having an internal cavity; a barrel affixed to the forward end of the receiver, the barrel having having a breech configured to receive ammunition and a muzzle, from which a projectile exits upon firing; protrusions from said upper or lower receiver mating surfaces which mate with aligned openings on the opposite upper or lower receiver; a bolt carrier disposed within the upper receiver cavity, the bolt carrier having a firing position and a retracted position defining a longitudinal axis of movement; one or more removable rail members mounted within the receiver and aligned with the longitudinal axis of movement of the bolt carrier, the rail members supporting and guiding the bolt carrier during movement of the bolt carrier; a transverse gas bore penetrating through the barrel through which high-pressure combustion gas passes upon firing, the gas bore being closer to the breech than to the muzzle; a gas block configured to receive high-pressure combustion gas from the gas bore; a gas piston being acted upon by said high-pressure combustion gas to generate a retraction force; a pushrod for transmitting the retraction force, the pushrod being mechanically connected to the gas piston and; a carrier key mechanically connected to said bolt carrier, receiving said retraction force from the pushrod, wherein the retraction force causes movement of the bolt carrier towards the retracted position.
 2. The improved firearm type firearm of claim 1 in which the coefficient of friction between the rail members and the bolt carrier is less than that of between the bolt carrier and the internal surface of the upper receiver cavity.
 3. The improved firearm of claim 1 in which the pushrod is formed as part of the gas piston.
 4. The improved firearm of claim 1 in which pushrod separates from the carrier key before the bolt carrier reaches the retracted position.
 5. The improved firearm of claim 1 in which the improved firearm is an M16-type firearm.
 6. An improved firearm, comprising: an upper receiver having an upper receiver cavity; a bolt carrier disposed within the upper receiver cavity and moving along a longitudinal axis between a firing position and a retracted position; and one or more rails mounted within the upper receiver, the rails supporting and guiding the bolt carrier during movement.
 7. The firearm of claim 6 in which the one or more rails are removable.
 8. The firearm of claim 6 in which the coefficient of friction between the one or more rails and the bolt carrier is less than that of between the bolt carrier and the internal surface of the upper receiver cavity.
 9. The firearm of claim 6 in which the bolt carrier and the upper receive include grooves for accepting a portion of the one or more rails.
 10. The firearm of claim 6 in which the one or more rails have circular cross sections.
 11. The firearm of claim 6 in which the one or more rails include a concave surface.
 12. The firearm of claim 6 in which the rails are removably affixed to the interior of the upper receiver cavity.
 13. The firearm of claim 6 in which the rails are affixed to the outer surface of the bolt carrier.
 14. The firearm of claim 6 in which the rails substantially reduce or eliminate the need for lubrication of the bolt carrier.
 15. The firearm of claim 6 in which the rails comprised a low-friction, high-temperature material.
 16. The firearm of claim 6 in which the rails comprise carbon or a carbon composite.
 17. The firearm of claim 16 in which the rails comprise polymer and chopped carbon fiber
 18. The firearm of claim 6 in which the rails comprise metal.
 19. The firearm of claim 6 in which the rails are disposed in longitudinal receiving areas machined into the bolt carrier and the upper receiver.
 20. The firearm of claim 19 in which the rails are retained within to the upper receiver housing by fixtures.
 21. The firearm of claim 20 in which the fixtures comprise any combination of: posts, bolts, screws, pins, clips.
 22. The firearm of claim 6 in which the improved firearm is an M16-type firearm. 23-38. (canceled)
 39. An improved autoloading M16-type firearm, comprising: a receiver, comprising an upper and lower receiver, said upper and lower receiver having mating surfaces and being separable from each other, the upper receiver having an internal cavity; a barrel affixed to the forward end of the receiver, the barrel having a breech configured to receive ammunition and a muzzle, from which a projectile exits upon firing; and one or more protrusions from said upper or lower receiver mating surfaces, said protrusions mating with aligned openings on the opposite upper or lower receiver.
 40. The firearm of claim 39, wherein the one or more protrusions are posts which protrude perpendicular to the mating surfaces.
 41. The firearm of claim 39, wherein the one or more protrusions are removable from the upper or lower receiver.
 42. The firearm of claim 39, wherein the one or more protrusions are integrated into the upper receiver and mate with corresponding aligned openings in the lower receiver.
 43. The firearm of claim 39, wherein the one or more protrusions are integrated into the lower receiver and mate with corresponding aligned openings in the upper receiver.
 44. The firearm of claim 39, wherein the one or more protrusions comprises protrusions on both the upper and lower receivers, with corresponding aligned openings on the opposite receivers.
 45. The firearm of claim 39, wherein the one or more protrusions on an upper or lower receiver are arranged such that they do not interfere with mating of the upper or lower receiver with a corresponding receiver which does not have aligned openings.
 46. The firearm of claim 39, wherein the one or more protrusions are monolithic protrusions. 